The pro-Hillary Clinton super PAC Ready for Hillary turned heads Sunday when it rented the use of the former Secretary of State’s old presidential campaign email list, sparking a new round of speculation that Clinton is moving closer to run once again for the White House. Left largely unremarked upon was the strategic reason that the deal was made.

Ready for Hillary, founded early last year to build a grassroots effort for a potential Clinton candidacy, is working to build a massive database of potential supporters should she decide to enter the race. They are assembling a massive list of names of Clinton supporters, with contact information, social media handles, volunteer information, issue priorities, contribution information, and even what products they purchased on the Ready for Hillary store. The group is then cross-referencing that data with the 50-state voter file — and they’re ready to hand it over to Clinton should she jump in.

In the past, such costly list-building activity has been done by campaigns themselves, but recent changes in campaign finance law, and evolving third-party strategies, have allowed outside groups funded by unlimited checks from wealthy interests to do much of the work of building campaign infrastructure. Federal election law prohibits campaigns and super PACs from coordinating on expenditures — television ads, direct mail, and the like. But there is nothing stopping them from sharing mailing lists or volunteer rolls, as long as they are compensated for them, according to Paul S. Ryan, senior counsel for the Campaign Legal Center. “A mailing list or email list itself doesn’t seem to fall within the scope of the coordination rules at all,” he told TIME. And of course, FEC rules don’t apply yet because Clinton is not yet a candidate.

Nonetheless, the Republican National Committee has tried to make political hay out of the tactics, accusing Clinton on Monday of “fully coordinating” with the outside groups.

If Clinton decides to run, her campaign can buy or rent the Ready for Hillary list at fair market value, though email marketing experts say pricing those lists is often flexible. Either way, the cost almost certainly wouldn’t cover what Ready for Hillary spent gathering the data. Alternatively, Ready for Hillary could simply encourage their signatories to sign up for Clinton’s campaign list once she enters the race, or the two organizations could swap records in a no-payment arrangement.